MortarThePoint

Sounds like people prefer the ceramic approach. Is there a specific reason to not like the corrugated metal approach? I know it needs replacing, but that would be after ~30 years. It's a lot easier to install if I understand correctly.

Lovely stone. A proper chimney may be beyond the scope of things at this stage so a flue liner is likely the only option. I suspect that means we have to have a log burning stove.

Definitely want to avoid that!

It would be for a log burner

The chimney has started going up which is some of the first above DPC brickwork so nice to see. It's thrown open a whole host of questions though, one of which is how to line the chimney (plans at the bottom). I see two main options: Ceramic (inc Pumice) flue liner: Tubular sections that are stacked on top of a concrete (?) base plate and sealed together. You surround the sections with either granular or fibre insulation. Very much built in to the chimney and needs to be done as the chimney goes up. I expect this would cost around £900 for 200mm x 7m. Here's a video that shows the concept: https://youtu.be/VVRhrdjoFbw Corrugated metal flue liner tube: available in 316 (15 years) or better 904 (30 years) grade stainless steel this costs about £340 for 200mm x 7m. I think these are double wall to warm up faster and I guess would just hang down the chimney from the top? It's certainly the cheapest choice, but also seems like the choice that is easiest to go back on and change to another approach at a later if wanted and doesn't need to be built in as the chimney goes up as far as I can see. We had planned to use a stove as I was under the impression that open fires were no longer allowed, but various suppliers are suggesting they are. If you had an open fire, I think you then need a gather that sits above the fire basket and would then attach to the flue liner, though I am uninformed there so if anyone has done this please share some knowledge. What approach did you choose?

Sounds like a plan. I'll also try to keep them on their toes but don't enjoy having to push people for their best work.

I asked my BCO and he said he'd prefer 1.6m max. I spoke to the brickies and they were concerned that the scaffolders wouldn't want to do their internal birdcage that low. I checked with the scaffolders and they said that was fine. All the rest will be stopping at 7 courses which I am happy about and the brickies are happy too as they won't have to visit each section twice before the first internal scaffold lift, they'll just be doing more off the internal scaffold (~1.6m to ~2.7m). To avoid snots in the cavity, the brickies are planning to have a timber at the top of cavity resting on the ties (to pull out vertically) and one at the bottom of cavity they can pull out sideways. I'm sure we are going to have words at some point about keeping the cavity clean, but hopefully it won't be too bad.

Yes that would help, but doesn't protect the ties from snots unfortunately.

I worded my initial post poorly. The brickies are planning to do all the inner leaf blockwork to 9 courses high. They then plan to do the outer leaf brickwork.

I've been pretty relaxed about it, but then spooked myself that perhaps I should have thought about it a bit more. I saw the NHBC guidance below that says not more than 6 block courses and they are going up to 9.

Interesting. I'd expect that any design would need to be tailored to the plot somewhat and various additional fees would push up the planning application cost. Depends how much of a struggle planning might be in terms of whether you'd need a complex design and access statement etc.

Sometimes what's difficult as a first time selfbuilder is when things differ from how you thought things would progress and being unsure if that's OK. I had expected inner and outer leaves to rise at the same time, but the brickies are planning to do all the inner leaf blockwork first and then the outer leaf brickwork. Is there anything wrong with this approach? I had expected the inner leaf to lag the outer leaf to help them keep the cavity clear, but as long as they manage to keep the cavity clean is there any other worry in the approach they are taking?

I might be missing something here. The gas membrane is at DPC level of the outer leaf and I am forming the DPC using the cavity tray that is on the surface of the membrane.

There is a company that makes some insulation for use with cavity trays: https://www.jackon-insulation.co.uk/products/detail/jackodur-cavity-tray-insulation/ I wouldn't be able to get these in time. I wondered about using polystyrene coving and pinning the cavity tray to the face of the inner leaf as then there would only be the curve as it makes a single transition from vertical to horizontal. Then I saw the coving is hollow so wouldn't help much. I'll have to make sure the cavity tray is installed loose enough for the blown beads to push it down onto the face of the inner leaf without applying a tension to the cavity tray and risking dislodging it. With a bit of care, it should be possible to have a radius of curvature of about 25mm or less at both the top and the bottom of the vertical portion of the cavity tray (as shown on the right side of my sketch) and so the missing amount of insulation will be small (about 10% over a height of 25mm). If the DPC material can actually be convinced to fold then it will be a lot less. 450mm is a standard width for DPC but that is a bit tight to make this out of. 102mm outer leaf, 100mm cavity, and 225mm up inner leaf leaves only 23mm stuck in to the inner leaf blockwork bed. Is that enough? If tight and crossing at an angle making maximum cold bridge, it would have about 100mm available for the inner leaf bed. I think I'll get my scissors out and trim down a load of 600mm wide DPC to 500mm.

We have a gas barrier membrane crossing the cavity and as I understand it the rule is that anything bridging the cavity has to have a cavity tray above it. It's a pain, but I think we have to have it.

Cavity trays made using DPC roll are often shown simplistically crossing the cavity at an angle from their top on the inner leaf to bottom on the outer leaf. In reality, even if you try to run the DPC material down the inner leaf before crossing the cavity, you will still create a void behind the cavity tray that will end up uninsulated: Is there a way to insulate this void or is it a matter of trying to keep it as small as possible? Preformed corners and stop ends all seem to adopt the shape on the left and so will create quite a significant cold bridge. I'm using blown bead insulation. There is no route for the beads to get between the gas membrane and cavity tray. Also I wouldn't want the pressure of the bead blowing to dislodge the cavity tray.

I found this LABC guidance: "The bottom of the tray should be taken across the external leaf and preferably just visible at the external mortar joint." That's preferably just visible, possibly not, and definitely not sticking out 10mm. This LABC guidance on the other hand is perhaps clearer: "5. Ensure the external edge of the DPC is visible and not bridged by mortar when completing pointing of the mortar joint."

Yes, but he's saying most of the mortar in the joint is above the DPC, rather than the DPC being smack in the middle.

Thankfully not, though I did consider it back at the time. BCO is fine with / expects the DPC recessed slightly

He does seem pretty young and inexperienced. He may be right 'on paper' though which makes it hard to swing him round or to get a second opinion from his firm / the warranty provider.

It's a mixture of brick and rendered block work with block work inner leaf. Gosh, they're even smaller. They won't have the "Venturi effect outlet tube that reduces water pressure, allowing for optimal drainage" if you believe that. It's quite a small opening for venting through. Do the TW1 work well?

The Invisiweep sticking 10mm proud or the standard ones. Either way, doesn't sound fantastic

Well I couldn't spot them in the photo so they clearly work ?

I require them all the way around the house at DPC level as I have a gas membrane crossing the cavity. Down there visibility may not be as much of a concern as pest proofing.

Welcome to the forum! Sounds like a nice project and there will be lots people can help you with here.